The glutamate receptor family is comprised of 18 genes that can combine to form dozens of different heteromeric receptor complexes. This receptor class mediates most excitatory synaptic transmission in the mammalian brain, and plays a central role in cellular models of learning and memory as well as a wide range of neuropathologies. Of these gene products, two orphan receptor subunits (delta1 and delta2) have been designated to be in the glutamate receptor family on the basis of sequence homology even though no activating ligand has been identified. The delta2 subunit is expressed at cerebellar synapses, and a variety of electrophysiological studies suggest that delta2 plays an important role in cerebellar development and synaptic plasticity. In addition, mice that either lack the delta2 subunit or express a constitutively active mutant form of this receptor (delta2-lurcher), show distinct phenotypes with motor disturbances. Thus, a paradox exists whereby the delta2 glutamate receptor subunit is thought to play an important role in brain function yet there is a vacuum regarding what we know about the receptor. Our collaborators have recently used crystallography to determine that a small amino acid (D-serine) can bind to the delta2 ligand binding domain, making a set of atomic contacts that are consistent with glutamate binding at other glutamate receptor subunits. Interestingly, although D-serine binds to the delta2 subunit, it does not induce any current response in wild- type receptors. However, a naturally occurring point mutation within the delta2 subunit (the lurcher mutation) produces constitutively active homomeric channels, and application of D- serine to these channels inhibits the tonic current flowing through these constitutively active channels. These data suggest that binding of D-serine can lead to changes in gating, and raises the possibility that incorporation of the delta2 subunit into heteromeric receptor assemblies may endow glutamate receptors with a regulatory site for D-serine. These new data provide an unprecedented opportunity to explore delta2 receptor structure, function, and pharmacology. We propose to exploit this recent finding in 2 lines of experimentation: Aim 1: Identify new ligands that potently activate or inhibit delta2 receptors. Aim 2: Evaluate whether co-expression of delta2 with other glutamate receptor subunits adds D-serine sensitivity to the receptors. Completion of these studies will provide the first comprehensive pharmacological evaluation of this receptor class, as well as new tools with which to study its function and regulation in brain. PUBLIC HEALTH RELEVANCE: Our recent findings that D-serine can bind to the orphan glutamate receptor delta2 has provided an opportunity to significantly advance our understanding of the function of this receptor, which has been enigmatic for over a decade despite in vivo findings that suggest an important role in cerebellar development and synaptic plasticity. The goal of these studies is to identify novel and selective pharmacological agents (activators and antagonists) that could be useful research tools for understanding delta2 function in tissue. In addition, we will also test whether delta2 subunit alters the effects of D-serine on hetero-mulitmeric glutamate receptors. Completion of these studies will advance our understanding of delta2 function, which holds implications for neuronal signaling, synaptic plasticity, and neuronal development in the cerebellum. These studies may also provide insight into new therapeutic agents that target the delta2 receptor.